Electrical discharge apparatus



Jan. 27, 1931. w, L 1,790,152

ELECTRICAL DISCHARGE APPARATUS Original Filed Oct. 13, 1922 2Sheets-Sheet 1 I Fig. 3

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A. W. HULL ELECTRICAL DISCHARGE APPARATUS Original Filed Oct. 15, 1922 2Sheets-Sheet 2 Wj {-52 {M Pig. 6

Inventor: Aibert W. Hui l,

His Tney Patented Jan. 27, 1931 UNITED STATES PATENT OFFICE ALBERT W.HULL, Oi SCKENECTADY, NEW YORK, ASSIGNOB TO GENERAL ELECTRIC COMPANY, ACORPORATION OF NEW YORK ELECTRICAL DISCHARGE APPARATUS Application filedOctober 13, 1922, Serial No. 594,370. Renewed September 18, 1930.

The present invention comprises an electrical discharge apparatus whichis suitable for general technical use and has certain novelcharacteristics which render the device particularly adaptable for powerconversion and control, as for example, the rectification of highpotential current, the breaking of high potential electric circuits, thedischarge of lightning and like technical 1.3 purposes.

Heretofore, commercial power devices containing cathodes operating atincandescence have been operated either in a vacuum so high that theionization of residual gas was negligible, that is, with a pure electrondischarge; or, in a gas at a pressure so high that an arc-like,concentrated discharge of negative volt-ampere characteristic hasresulted.

In pure electron discharge devices the conduction of current isaccompanied by a high voltage drop in the discharge path, caused by themutual repulsion of the electrons on each other, which is known as thespace charge effect. This high space charge has confined power devicesoperatin by pure electron conduction to the field o relatively smallcurrents at high voltage.

Gaseous conduction devices containin in- 0 candescent cathodes on theother hand ave been restricted heretofore, by certain inherentcharacteristics, to a low voltage field and have been used chiefly forthe rectification of alternating currents of moderate amperage andvoltage. In some low-powered relay devices, such as radio detectors, agas at low pressure has been provided to increase the sensitivity. Inthe power field, however,

the utilization of gas in electronic devices in amounts suflicient onlyto reduce or eliminate space charge by positive ionization withoutproducing an arc-like discharge has been excluded for various reasons,such as the irregular disappearance of the gas by an electrlcal effect,disintegration or sputtering of the cathode, deleterious arcing, and soforth. As a consequence of my invention these difiiculties have beenovercome and I have provided a new gas ionization electron dischargedevice which is radically unlike the arc type of incandescent cathodedevices and whic is capable of use for power purposes, as for examplethe control of power currents and the rectification of alternatingcurrents.

One of the new'properties characteristic of devices embodying myinvention is the high current-carrying capacity which is maintained whenthe voltage impressed between the electrodes of the device is chosen ata value above the ionization voltage of the as filling and below acritical value, but Wl'llCh drops substantially to zero when the voltageimpressed on the device for any reason rises above this range.

The essential features of my invention comprise a cathode containing amaterial capable of forming a surface film of high electron emissivityand a gas or gasifiable material which is inert with respect to thecathode film and which is present at a pressure sufficiently high toreduce space charge by being ionized.

The geometric or spatial relation'of the electrodes is so chosen thatsubstantially all the positive ions strike the heated cathode, insteadof being permitted to discharge themselves upon the container walls, orother cooler parts, which would cause consumption of gas by thedisappearance of ions into the body of the glass container or otherdischarging surface. When these conditions are fulfilled, heavy currentsmay be conducted at high efficiency without substantial clean up of gas.Sufiicient gas or vapor is provided to materially reduce or'eliminatespace charge, the pressure preferably being materially below the regionof pressure at which the discharge becomes arc-like in character, andbecomes concentrated or localized upon the cathode. This pressureordinarily is within the range of about several microns up to as high asseveral millimeters of mercury, depending on the nature of the as. g Inaccordance with another feature of my invention, I have provided arectifier capable of rectifying currents over a wide range of voltagesby so arranging the discharge-carrying electrodes that the maximumdistance from anode to cathode in the direction of an electric field,produced by an applied potential, is less than the mean free path of theelectrons in the gas at the given pressure of gas in the device.Preferably the anode entirely and closely surrounds the heated portionof the cathode thereby fulfilling both aforesaid conditions.

. I have discovered that various advantages result when the cathode isconstituted of a material capable of forming a surface film of highelectron emissivity, as for example, a cathode of the thoriated type,one form of which is described in Langmuir Patent 1,244,216. Inaccordance with my invention the utilization of the thoriated type ofcathode has been extended from the field of discharges operating withoutsubstantial ionization to the field of devices making use of gasionization during operation. In this case, the device is capable ofconducting substantial currents, say of the order of magnitude of threeamperes per sq. c. m. of cathode surface, when the temperature of thecathode is about 1800 C., provided the voltage between the electrodesdoes not exceed a critical value, which depends on the character of thegas and varies for about 10 to 50 volts in difierent gases. At materallyhigher impressed voltages a sudden reduction of electron emissivityoccurs, the high conducting condition being restored when the highvoltage is removed. The latter unique function of my new devices rendersthe same applicable to various special purposes, as, for example, ascircuit breakers or as lighting arresters.

The accompanying drawing shows in Fig. 1, partly in section, oneembodiment of my invention; Figs. 2 and '7 are diagrammatic sectionalviews of modifications; Fig. 3 is a diagram of a current-voltagecharacteristic of a device embodying my invention Figs. 4, 5 and 6 arediagrams illustrating technical applications of my invention.

The device shown in Fig. 1 comprises a tubular glass envelope 1, uponthe reentrant stems 2, 3 of which is supported an anode 4. Thethermionic cathode 5 is supported axially within the cylinder andconsists preferably of tungsten containing a small amount of thoria,that is, of the order of one per cent of thoria, together with aboutone-half per cent of a reducing agent, such, for example, as carbon. Insome cases magnesium or calcium may be used in place of or in additionto carbon. As described in Langmuir Patent 1,244,216, a thoriatedcathode when in a con dition of high electron emissivity, functions witha surface film or coatin of thorium, or possibly some compound ofthorium, in a lower state of oxidation than thoria.

The cathode conductors 6, 7 are sealed into the stems 2, 3 in the usualmanner. The helical spring 8 maintains the filament 5 taut duringoperation of the device. The anode conductor and support 9 has beenscaled into a side arm 10, but of course can be otherwise supported. Theenvelope contains a gas which is inertwith respect to the cathode underoperating conditions, and which is stable when ionized, a gas of therare or monatomic group being preferred. For ex ample, the envelope maycontain argon or noon at a pressure of about 20 to 100 microns ofmercury. In some cases, especially using a thoriated filament cathode,sodium or potassium in the gaseous state may constitute the ionizablemedium, the tempera ture of the device being so chosen that a suitablegaseous pressure of vaporized alkali metal prevails in the tube. Thecathode is heated during operation by a suitable current supply (notshown), connected to the conductors 6, 7.

It will be noted that in the structure illustrated, the anode 4 entirelyencloses the cathode 5 so that the positive ions formed by theionization by collision of the electrons with gas molecules arerestricted to the space between the electrodes and therefore areprevented from discharging on the glass container orany part of thedevice itself other than the cathode, which, being negatively charged,attracts the positive ions. I have found that this discharge of the ionsupon an incandescent surface does not clean up or fix gas, therefore,the gas pressure within the device is maintained substantially constantduring use.

lVhen the device is to be used as a rectifier of alternating currents,the geometric rela tions of the electrodes and the pressure of thegaseous filling should be so chosen that the number of collisionsbetween electrons passing from cathode to anode and gas moleculesproduces sufiicient ionization of gas to eliminate, or at least tosubstantially reduce, space charge, but the electrons also should beprevented from taking such a long path from anode to cathode in thedirection of an electric field produced by the applied potential as topermit a discharge during the half cycle intervals when the anode isnegative. In other words, with the gas pressures here involved thelonger the path an electron may take in its passage from one electrodeto another, the lower the voltage required to produce a reversedischarge from the unheated electrode to the heated electrode. It is notonly nccessa r that the cathode be located close to the anode but it isalso necessary that the electrons from the electrode which is normallyanode should be prevented from taking a round about path from theoutside of the anode to the cathode travelling in the dire tion of theelectric field.

\Vith a pressure of 30 microns of argon a tube having a dimensionbetween the anode and cathode of about 1.25 c. m., and being constructedas shown in Fig. 1 so that the heated part of the cathode does notproject beyond the anode and the mouth of the anode is closed, willrectify currents at 8000 volts or higher, without an inverse dischargefrom anode to cathode.

During the rectification of current at this high voltage, most of thevoltage is consumed in an external load which is indicated by crosses inFig. 1, and during the passage of current the voltage drop in the tubeitself must be less than a critical voltage which depends on. the natureof the gas. In argon gas at about 30 microns pressure, the criticalvoltage is about 20 volts which is but slightly above the ionizationvoltage of argon. In creasing the potential between the electrodes 4 and5 above the critical voltage, for example, by short-circuiting the loadwholly or in part, results in a decrease of electron emis sion to a lowvalue due to the removal of the active film of thorium from the cathodesurface by positive ion bombardment.

Fig. 3 is a curve showing the current-voltage characteristic of a deviceembodying my invention when different voltages are steadily applieduntil an equilibrium condition is attained. The current rapidlyincreases with the applied voltage up to a maximum value indicated bythe dotted line 31. The voltage corresponding to this current valueherein has been called the critical voltage. When the applied voltagerises above this critical value, the current rapidly decreases and, at avalue several times the critical voltage the current falls substantiallyto zero.

My device, therefore, is an ideal circuit breaker and may perform thisfunction at the same time that it acts as a rectifier. When theconductors 11, 11 (Fig. 1) are connected to a source of alternatingcurrent, the impedance of the load should he so chosen with respect tothe electrical characteristics of the tube, that the voltage across theelectrodes 4, 5 during normal operation does not exceed the criticalvoltage.

Should the critical voltage be exceeded, for example by a short circuitin the load, a device is provided for automatically opening the circuit,thus removing the voltage impressed on the electrodes, so as to enablethe electron emissivity of the cathode to be restored. This device mayassume the form of a circuit breaker 34 having a series or currentwinding 35 and a high resistance voltage winding 36. During normaloperation these two windings pull in opposite directions, the

contact 34 remaining closed. Should the current fall to a low value byreason of the loss of" emissivity of the cathode, the coil 35 becomesineffective and the circuit breaker opens the circuit by the action ofthe coil 36. The opening of the circuit deenergizes the coil 36 andgravity closes the circuit. During the interval of open circuit thecathode regains its high emissivity by the formation of a sensitivefilm. Preferably the closing of the circuit breaker is delayed by adashpot 37, or other suitable device, to give suflicient time for thecathode to regain its high emissivity. .In case the short circuit stillexists, the operation will be repeated without harmful effect.

I have shown somewhat diagrammatically in Fig. 2 a modified dischargetube embodying my invention operating as a rectifier from a polyphasesource. As shown in a section taken at right angles to the main axis ofthe device, there are provided a plurality 0t anodes 12, 13, 14, 15, 16and 17, spaced about a central cathode 18 in an envelope 19. The cathodeconsists of an axial filament, such as shown in Fig. 1, and the anodesconsist of plates extending the length of the cathode.

and grouped about the sameas indicated in Fig. 2. The envelope containsa suitable gas at a pressure high enough to remove space charge, butbelow the pressure at whichconcentration of the discharge occursupon'the cathode surface.

The anodes 12 and 13 are connected by the conductors 18' and 18 to thesecondary of a transformer 20; the anodes 14, 15 are con nected by theconductors 21, 22 to the secondary of a transformer 23; and the anodes10, 17 are connected by the conductors 24, 25 to the secondary of atransformer 26. The primaries of these transformers 20, 23 and 26 areconnected in delta and are supplied with three phase alternating currentby the conductors 27, 28 and 29. A direct current motor 30 has beenshown for illustrative purposes as a load device.

In case of a short-circuit of the load 30, the rectifier tube acts as acircuit breaker by the removal of the active film on the cathode, asdescribed above. During normal operation, the contactor 32 is heldclosed by a series coil 33. When a short-circuit occurs and the currenttransmitted by the tube falls to a low value, the series coil isdeenergized and the contactor opens. This removes the voltage from therectified tube and the cathode again becomes coated with a sensitivelayer of high electron emissivity ready for operation. For the sake ofsimplicity the contactor in this case has been shown in a simple formwhich may be closed by hand.

I have shown in Figs. 4, 5 and 6 several technical applications of myinvention. The discharge tube is diagrammatically indicated in thesefigures by a hair-pin shaped cathode, surrounded by a U-shaped anode.Although for lightningarrester purposes the discharge device may be ofthe type shown in Fig. 1, it is not necessary for this purpose to employthis particular structure for the electrodes.

Fig. 4 illustrates my invention used as a lightning arrester, meaning bythe term lightning to include surges or other high volt-age disturbancesof short duration. The discharge tube itself has been symbolized at 40,The filament located. within the anode is 1 indicated as being heated bya battery, the enclosing envelope not being shown. The device 40 isshown as connected between a line 41 and ground in series with a spheregap 42. High voltage discharges of short duration will pass to earththrough the tube without raising the voltage across the terminals abovethe critical voltage, but the line current is prevented from followingas the passage of a considerable current will raise the voltage acrossthe tube above the critical value and will render insensitive thecathode, or, in other words, cause it to lose its condition of highelectron emissivity, thereby interrupting the current. Hence, as soon asthe lightnin discharge has passed the sphere gap again insulates theline from ground, and the film quickly reforms on the cathode.

Fig. 5 shows an application of my new device to the interruption ofalternating current. The conductors 43, 44 are fed by an alternatingcurrent generator 45. The load represented by a motor 46 is connected tothe secondary of a transformer 47, the primary of which is connectedacross the supply lines 43, 44, in series with the devices 48, 49oppositely connected; that is, the anode of the device 48 and thecathode of the device 49 are connected to the conductor 43, the cathodeof the device 48 and the anode of the device 49 are connected to theprimary of the transformer 47. The circuit conditions are so chosen thatthe electron tubes normally operate below their criticalvoltages.

When the load is taking current, the switches 50, 51 in the cathodeheating circuit are closed, heating the cathodes to incandescence. Whenthese switches are opened, the cathodes become deenergized and begin tocool, hence diminishing their electron emissivity. This decreases thevoltage drop in the load, and consequently increases the voltage drop inthe electron tubes. As the voltage in the tubes rises above the criticalvoltage, the cathodes rapidly lose emissivity by the destruction of thesurface film, thereby still further reducing the current until thecircuit is open. By the rapid decrease in cathode emissivity, theliberation of large amounts of energy in the electron tubes, as thecurrent falls, is avoided and the current is interrupted smoothlywithout surges. If desired, a switch 52 may be provided to finally cutall of the devices out of circuit.

Fig. 6 is a direct current circuit illustrating the employment of my newdevice as a switch or circuit breaker only at the time the circuit is tobe opened.

The load 53 is connected across the lines 54, 55 fed by a direct currentgenerator 56. When the load 53 is taking current the switch 57 is closedand the switch 58 is open. When the circuit is to be opened, the switch58 is closed, energizing the cathode of the device 59. The switch 57thereupon is opened, inserting the device 59 into circuit. The switch 58is then opened, deenergizing the cathode and interrupting the circuit asalready explained above. A switch 60 may be provided to permanently openthe circuit after it has been deenergized.

In the device shown in Fig. 7 the enclosing envelope is constituted inpart by the cylindrical anode 62 which is sealed at opposite ends tosomewhat tapered sleeves 63, 64 consisting of copper-coated, nickel-ironalloy adapted to make a gas-tight seal with glass. The sleeves 63, 64 inturn are joined to tubes 65, 66 consisting of suitable refractory glass.The glass tube 66 is sealed by fusion to a nickel-iron sleeve 67. Thesleeve 67 is joined by brazing, or other convenient way, to a resilientdiaphragm 69 of sylphon metal or other suitable extensible material.This diaphragm is joined to the cap 70 through which projects a cathodeconductor 71 making a gas-tight seal. This diaphragm provides means forlinear displacement between the anode 62 and the cathode conductor 71.The opposite cathode conductor 72 is suitably sealed into another cap 73 which in turn is joined to the glass tube 65. The cathode 68 consistsof thoriated tungsten, or other suitable material of hi h electronemissivity. The anode 62 re erably is cooled artificially, for examp e,by a blast of air, fins 74 preferably being rovided to increase heatdissipation. The tube may be evacuated and supplied with a suitable gasfilling by a tube 75.

The construction of the vacuum discharge device shown in Fig. 1 of thisapplication is also shown in applicants copending application, SerialNo. 226,276, filed October 15, 1927, as a continuation in part of thepresent application. In said continuation applica tion other examples oflow pressure electron discharge devices are also disclosed embodying thefundamental feature of operation above the ionization voltage and belowthe disintegration voltage and applicant has chosen to put the broadclaims in said continuation case, the claims of the present applicationbeing drawn to the form of discharge tube disclosed in the present caseand to the circuit breaking features of the device.

What I claim as new, and desire to secure by Letters Patent of theUnited States, is

1. An electrical discharge device comprising an envelope, a thermioniccathode therein, rmeans for heating said cathode, an anode, and a gas insaid envelope which is inert with respect to said cathode and has apressure sufliciently high to permit substantial ionization but beingbelow the pressure at which the discharge becomes localized upon thecathode, the parts of said device being geometrically so constructedthat positive ions produced during operation can strike substantiallyonly the heated part of the cathode.

2. An electrical discharge device comprising a thermionic cathode, ananode surrounding said cathode, and a material in the discharge spacebetween said electrodes which is gaseous at the operating temperature ofsaid device and which will have a pressure, sufficiently high toneutralize space charge but insufiicient to produce a concentrated orarc-like discharge at the operating temperature, the distance betweensaid electrodes in the direction of an electric field therebetween beingof the order of magnitude of the mean free path of electrons in saidgas.

3. The combination of an electrical discharge device having a thermioniccathode, means for forming on saidcathode during operation a film havinghigher electron emissivity than said cathode but being sensitivepositive ion bombardment at voltages above a critical voltage, and amechanical circuit breaker having an operating coil connected across theterminals of said device and having a make and break switch connected inseries with said device.

4. An electron discharge device comprising an electrode which is capableof emitting electrons when heated, means for providing in the path ofsaid electrons at operating temperatures of said device a gas at apressure sufliciently hi h to eliminate space charge by ionization an asecond electrode enclosing said cathode end spatially so related theretothat the positive ions formed by the ionization of said gas strike thecathode only, the maximum distance between said electrodes in thedirection of an electric field, when potential is applied between saidelectrodes, being less than the mean free path of electrons in said gas.

5. An electron discharge device comprising a container, a filamentarythermionic cathode therein adapted to form during operation a surfacelayer capable of a'higher order of electron emissivity than the mainbody of said cathode, a material in said container chemically inert withrespect to said layer and having during operation a gaseous pressure sufficiently high to ermit substantial ionization materially to increasethe current-carrying capacity of said device but not so high as to causeconcentration of the discharge locally on the cathode and an anodesubstantially completely enclosing said cathode and being geometricallyso constructed that positive ions produced during operation can strikesubstantially only the said filamentary cathode and are substantiallyprevented from striking the walls of said container.

6. An electrical discharge device comprising an envelope, electrodestherein including a thermionic cathode and an anode, means for heatingsaid cathode, and a gas in said envelope which is inert with respect tosaid cathode and has a pressure within the limits of about one micronand one millimeter of mercury to permit substantial ionization to reducespace charge, and operation with a discharge which is not localized uponthe cathode, the parts of said device being geometrically so constructedthat positive ions produced during operation can strike substantiallyonly heated electrode parts.

7 An electron discharge device comprising an incandescible cathodecapable of forming a surface film of thorium, an anode substantiallyenclosing said cathode and a gas in said container chemically inert withrespect to said cathode at a pressure sufliciently high to neutralizespace charge in said device when said gas is ionized but not so high asto cause concentration of the discharge on the cathode surface.

8. An electric high voltage rectifier comprising a sealed container, athermionic cathode adapted to form during operation a surface filmcapable of a higher order of electron emission than the body of saidcathode, means for heating said cathode, an anode substantiallyenclosing said cathode and conductive parts connected to said cathode sothat positive ions produced during operation of the rectifier can strikesubstantially only the heated part of said cathode and a gas in saidcontainer at a pressure sufficiently high to permit substantialionization to increase the current carrying capacity of said rectifierwhen said gas is ionized but below the pressure at which an arc-likedischarge becomes localized on the cathode.

9. The method of operating an electric discharge through a gas betweenan anode and an incandescible cathode coated with a surface layer ofhigh electron emissii ity, said anode and cathode being included in anexternal circuit, which consists in maintaining during runningconditions the voltage difference between said electrodes above theionization voltage of said gas and below a critical value depending onthe nature of the gas at which a decrease of electron emission occurs byremoval of said coating, and raising the voltage difference when saidcircuit is to be opened to a value materially above said critical value.

10. The method of operating an electric discharge with an impressedvoltage materially in excess of fifty volts in a gas having a pressurewithin the range of several microns to several millimeters of mercurypressure between cooperating electrodes including a thermionic cathodewhich consists in consuming sufiicient voltage external to said cathodeduring normal operation to maintain the ion bombardment voltage to whichsaid cathode is subjected below a critical voltage not exceeding fiftyvolts at which deleterious sputtering of said cathode would occur, andwhen interruption of the circuit is desired causring the voltage drop atthe cathode to increase materially above said critical voltage.

11. An electrical discharge device comprising an envelope, a thermioniccathode therein, means for heating said cathode to a temperature ofelectron emissivity, a gas in said envelope which is inert with respectto said cathode and has a pressure sufiiciently high to permitsubstantial ionization but not sufiicient to cause the discharge tobecome localized upon the cathode, and an anode so constructed andspaced from the cathode that the maximum distance from anode to cathodein the direction of an electric field produced by an applied potentialis less than the mean free path of electrons in the gaseous filling. Inwitness whereof, I have hereunto set my hand this 12th day of October,1922.

ALBERT W. HULL.

